In mechanical-drive helicopters, the lifting rotor is connected via a vertical transmission shaft to a speed reduction gear ensuring the transmission of power from the propulsion engine or engines of the apparatus. The assembly of the rotor-shaft-reduction gear constitutes, with respect to the structure of the helicopter, a relatively rigid assembly.
The speed reduction gear constitutes the main gear box and the case of each box is generally fixed on the structure of the helicopter by two types of link. The first link comprises a certain number of rigid elements, such as bars or stiffeners, in a number at least equal to three, interposed obliquely between the upper part of said case and strong points of the structure of the fuselage. The second link is a direct link between the lower part of this case and said structure; it is intended to take up the countertorque of the rotor, totally or partially. On the other hand, the functioning of said rotor for lifting and propelling the helicopter produces a vibration excitation essentially in the plane of said rotor. To attenuate its transmission to the fuselage, there may be disposed between the lower part of the case of the gear box and the fuselage, elements flexible in translation in a plane parallel to that of the rotor, but having considerable ridigity in rotation about its axis so as to be in a position to transmit the countertorque of the rotor.
U.S. Pat. No. 3,502,290 describes such an elastic link. It is formed by a flexible mounting plate constituted by a plate in which certain zones have cut-out portions, in the manner of a grid comprising bars. The flexibility thus obtained allows filtering of the vibrations by elastic deformation of the mounting plate at least in a direction thus privileged.
Other arrangements of suspension plate between the lower part of the main gear box and the top of the fuselage are described in U.S. Pat. No. 3,920,202 and in U.S. patent application Ser. No. 21,778 of Mar. 19, 1979 now U.S. Pat. No. 4,274,510. These mounting plates make it possible both to obtain, in their plane, a flexibility in mono- or multidirectional translation and to transmit the torque induced by the gear box on the fuselage.
All these devices ensure a relatively effective filtering of the vibrations produced by the rotor. However, on modern fast helicopters, the desired improvement in comfort imposes a search for a still lower vibration level. The corresponding attenuation of the vibrations may be obtained by means complementary of the preceding systems. For example, resonator devices employing blades or springs placed at different points of the helicopter fuselage and possibly on the flight controls may be used. These resonators oscillate at their own frequency, with such amplitude and phase that they absorb, at least partially, the alternate efforts transmitted by their points of fixation. These devices present, however, major drawbacks. On the one hand, such resonators must be disposed at each of the points where vibrations must be attenuated; this results in a considerable weight excess. On the other hand, the effect of attenuating the vibrations, obtained with each resonator, is limited to the vicinity of its point of fixation. Finally, the positioning of a resonator at a determined point may have for effect to amplify the vibrations at other points of the apparatus.
To overcome these drawbacks, resonators have been made, disposed above the hub of the rotor, from which they receive their excitation directly. These resonators may be constituted by a flapping weight, returned into neutral position by antagonistic springs. Such resonators are for example the subject matter of U.S. application Ser. No. 9614, now U.S. Pat No. 4,255,084, and No. 9578, now U.S. Pat. No. 4,281,967 of Feb. 5, 1979. These resonators offer the advantage of opposing the vibrations as near as possible to their source, i.e. on the rotor hub itself. They are all the more efficient as they are used in complement to the devices for suspension of the lower part of the main gear box, such as those described in U.S. Pat. Nos. 3,502,290 and 3,920,202.
However, such antivibration devices (hub resonators and elastic links between gear box and fuselage), although they generally enable a very good vibration level to be obtained on helicopters having more than two blades on the main rotor, present the drawback of a relatively considerable weight and also of a high cost price.